REFERENCE SEDIMENT-TRANSPORT RATES FOR LEVEL III ECOREGIONS AND PRELIMINARY LINKS WITH AQUATIC INDICIES

Sediment is listed as one of the principle pollutants of surface waters in the United States, both in terms of sediment quantity (“clean sediment”) and sediment quality due to adsorbed constituents and contaminants. We can view sediment-transport rates and amounts as (1) “natural” or background, resulting from generally stable channel systems, (2) “impacted”, with greater transport rates and amounts, reflecting a disturbance of some magnitude and more pervasive erosion, and (3) “impaired”, where erosion and sediment transport rates and amounts are so great that biologic communities and other designated stream uses are adversely effected. Impairment of designated stream uses by clean sediment (neglecting adsorbed constituents) may occur through processes that occur on the channel bed or by processes that take place in the water column. Fully mobile streambeds, and deposition of fines amidst interstitial streambed sands and gravels can pose hazards to fish and benthic macro-invertebrate communities by disrupting habitats, degrading spawning habitat, and reducing the flow of oxygen through gravel beds. Although lethal or sub-lethal thresholds are unknown at this time, high concentrations of suspended sediment, perhaps over certain durations can adversely affect those aquatic species that filter and ingest water. It is critical, therefore, to clearly identify the potential functional relation between an impact due to sediment and the sediment process so that appropriate parameters are analyzed.